As disclosed in JP-A-2008-298803 (patent document 1), an in-cylinder pressure sensor is mounted on an internal combustion engine to output a signal corresponding to a pressure inside a combustion chamber of a cylinder (in-cylinder pressure) so that a combustion stage in the combustion chamber is detected based on a detection result of the pressure sensor. An in-cylinder pressure sensor body is fitted in an insertion hole formed in a cylinder head so that the pressure in the combustion chamber is applied to a piezoelectric transducer through a diaphragm of the pressure sensor. The pressure sensor thus outputs, as the output signal, a voltage corresponding to the applied pressure. As a sensor characteristic, an offset voltage is outputted when atmospheric pressure of a predetermined pressure level is applied.
Further, as disclosed in JP-A-7-280686 (patent document 2), an engine electronic control unit (ECU) detects an in-cylinder pressure by acquisition and A/D-conversion of an output signal of an in-cylinder pressure sensor. A combustion state such as ignition timing or combustion temperature is determined so that knocking or misfire is detected based on the detected in-cylinder pressure.
Patent document 2 further discloses a configuration for inputting the output signal of the pressure sensor to a microcomputer through an A/D conversion circuit after amplification by an amplification circuit in case of detecting the pressure by the pressure sensor.
It is not possible to eliminate a drift of the offset voltage of the pressure sensor caused by factors such as temperature under a surrounding environment such as in an internal combustion engine, in which temperature changes rapidly. It is therefore essential to correct the drift to accurately determine the pressure. In patent document 2, the drift of the output voltage is corrected by resetting the output voltage of the pressure sensor to a reference value at a timing, which corresponds to a predetermined crank angle based on a detection signal of a crank angle sensor provided for the internal combustion engine.
According to the above-described correction technology, it is necessary for a microcomputer to transmit an instruction to the pressure sensor side to reset the output signal of the pressure sensor to the reference value. For this reason, the microcomputer and the pressure sensor need be connected by a signal wire, which is separate from a signal wire of the pressure sensor for transmitting the output signal. This correction technology thus increases costs.